Hybrid Electric Vehicle Controller and Method of Controlling a Hybrid Electric Vehicle

ABSTRACT

Embodiments of the invention provide a controller for a hybrid electric vehicle (HEV) having an engine and an electric machine, the controller being configured upon start-up to control an electric machine to provide torque to drive a vehicle with an engine off if a state of charge (SoC) of an energy storage device is above an EV-start SoC threshold and to start an engine if a SoC of an energy storage device is below the EV-start SoC threshold, wherein the EV-start SoC threshold is determined to be one selected from amongst a value sufficient to allow a vehicle to travel a prescribed distance before a SoC falls below a SoC minimum level at which an engine is started and a value sufficient to allow a vehicle to operate for a prescribed time period before a SoC falls below the SoC minimum level.

FIELD OF THE INVENTION

The present invention relates to a controller for a hybrid electricvehicle (HEV), a HEV and a method of controlling a HEV. In particularbut not exclusively the invention relates to a controller for aparallel-type HEV, to a parallel-type HEV and to a method of controllinga parallel-type HEV during starting of the vehicle. Aspects of theinvention relate to a controller, to a method and to a vehicle.

BACKGROUND

It is known to provide a hybrid electric vehicle (HEV) having anelectric machine and an internal combustion engine (ICE) connected inparallel to a driveline of the vehicle.

The vehicle may be operated in an electric vehicle (EV) mode in whichtorque to the driveline is provided exclusively by the electric machine.Alternatively the vehicle may be operated in a parallel mode in whichtorque is provided to the driveline by the ICE and electric machine.

STATEMENT OF THE INVENTION

Aspects of the invention provide a controller, a vehicle and a method asclaimed in the appended claims.

According to another aspect of the invention for which protection issought there is provided a controller for a hybrid electric vehicle(HEV) having an engine and an electric machine, the controller beingconfigured upon start-up to control an electric machine to providetorque to drive a vehicle with an engine off if a state of charge (SoC)of an energy storage device is above an EV-start SoC threshold and tostart an engine if a SoC of an energy storage device is below theEV-start SoC threshold, wherein the EV-start SoC threshold is determinedto be one selected from amongst a value sufficient to allow a vehicle totravel a prescribed distance before a SoC falls below a SoC minimumlevel at which an engine is started and a value sufficient to allow avehicle to operate for a prescribed time period before a SoC falls belowthe SoC minimum level.

It is to be understood that by ‘start-up’ is meant a transition of thevehicle from a condition in which the vehicle is not ready to move underits own power to a condition in which the vehicle is ready to move underits own power. For example in embodiments having a transmission andaccelerator pedal, upon start-up the vehicle is ready to move under itsown power (by means of the electric machine and/or the engine dependingupon the type of hybrid vehicle, e.g. parallel or series) if thetransmission is engaged and the accelerator pedal is pressed.

Some embodiments of the invention have the advantage that becausestart-up in EV mode is performed when conditions permit the likelihoodof start-up occurring in EV mode is increased. This has the advantagethat a vehicle may provide a more consistent start-up experience for adriver of the vehicle.

Embodiments of the invention have the advantage that a risk that thedriver of the vehicle is disappointed that the vehicle only travels arelatively short distance in EV mode in some embodiments, or for arelatively short time period in other embodiments, before the engine isstarted, may be reduced. Furthermore a noise, vibration and harshnessperformance of the vehicle may be enhanced since NVH associated withengine start is more likely to occur either upon initial start-up of thevehicle or after the vehicle has travelled a prescribed distance. Inaddition, embodiments of the invention allow a more consistent ‘feel’ tooperation of the vehicle to be maintained. This is in part because thevehicle is more likely to start in EV mode than known vehicles.Furthermore, a risk of distraction of an operator or persons around avehicle due to starting of an engine soon after launch of the vehicle isreduced.

These factors can be important since a vehicle is often required toperform manoeuvres in relatively confined spaces upon launch, e.g. whenreversing from a parking bay, manoeuvring from a parallel-parkedposition, exiting a driveway or parking garage, entering a parking rampand other similar situations.

Furthermore, in the case of off-road driving it may be distracting to adriver if the engine starts when the driver is performing a particularmanoeuvre such as descending a slope or driving through a narrow gap.

It is to be understood that if the vehicle travels at a relatively lowspeed, the time between start-up of the vehicle and engine start will belonger than if the vehicle travels at a relatively high speed. However,NVH performance of a vehicle is typically degraded by engine start to alesser extent at high speed than low speed due to the higher levels ofNVH associated with higher speeds. Thus an operator of the vehicle isless likely to notice NVH associated with engine start when the vehicleis travelling at speed, and therefore be less concerned that the vehiclehas travelled only a relatively short distance before starting theengine.

Furthermore, embodiments of the invention have the advantage thatbecause the vehicle attempts to travel by means of torque provided bythe electric machine with the engine off by default, nuisance noiseassociated with starting of the engine and initial driving of a vehiclein a residential area may be reduced. Thus a driver departing from aresidential area early in the morning enjoys a reduced risk ofdisturbing residents of the area.

Thus embodiments of the invention may provide a significant enhancementto a level of driver enjoyment of a vehicle.

Advantageously the controller may be operable to determine the value ofthe EV-start SoC threshold responsive to one or more vehicle stateparameters, said one or more state parameters being indicative of astate of a vehicle, said one of more state parameters being indicativeof an amount of charge an energy storage device is likely to be requiredto provide to drive a vehicle with an engine off.

Further advantageously the one or more vehicle state parameters may beselected from amongst an amount of current being drawn from a battery,an operational state of one or more vehicle accessories, a state of oneor more electrical heating functions, an ambient light level, a state ofone or more vehicle running lights, a vehicle loading and adetermination whether a vehicle is towing a load.

Thus it is to be understood that the controller may be configured todetermine a distance a vehicle is likely to be able to travel or a timeperiod for which a vehicle is likely to be able to operate responsive tothe one or more vehicle state parameters.

Advantageously the controller may be operable to determine the EV-startSoC threshold responsive to historical data in respect of a journeyconducted by a vehicle.

The controller may be operable to determine the EV-start SoC thresholdresponsive to energy consumption data in respect of at least a portionof a previous journey made by a vehicle.

Optionally the controller is operable to determine the EV-start SoCthreshold responsive to data in respect of a geographical location of avehicle.

Advantageously the controller may be operable to determine the EV-startSoC threshold responsive to data in respect of at least one selectedfrom amongst an average speed of a vehicle on a route from a currentgeographical location of a vehicle, a speed limit associated with aroute of a vehicle from a current geographical location and terrain datain respect of a route of a vehicle from a current geographical location.

Thus the vehicle may be operable to determine a likely charge drain onan energy storage device in order to control a vehicle to travel at anaverage speed of a vehicle on a route from a current location or totravel at a speed limit of a vehicle on a route from a current location.In some embodiments the controller may be configured to determine alikely charge drain taking into account whether a vehicle must negotiateone or more slopes on one or more routes from a current location.

Optionally the prescribed distance or prescribed time period is itselfresponsive to one or more parameters.

Advantageously the prescribed distance or prescribed time period may beresponsive to a geographical location of a vehicle.

Further advantageously the prescribed distance or prescribed time periodmay be responsive to a determination whether a vehicle is located in anurban or non-urban area.

The prescribed distance or prescribed time period may be set to lowervalues in an urban area thereby to increase a likelihood of a vehiclestarting a journey in EV mode.

It is to be understood that in some situations if a vehicle is locatedin an urban area, an average speed of a vehicle may be expected to belower than operations in non-urban areas. Consequently a rate of chargedrain on an energy storage device may be expected to be lower over agiven time period or over a given distance travelled.

Alternatively the prescribed distance or prescribed time period may beset to higher values in an urban area. An average speed of a vehicle maybe lower in an urban area and therefore when an engine is started NVHassociated with engine starting may be more noticeable to a driver whenin an urban area. Furthermore, in some situations it is desirable tomaximise an amount of time for which an engine is switched on in anurban area. Thus a lower limit of an allowable state of charge of anenergy storage device (the minimum SoC level)

Advantageously the SoC threshold may be set to a value sufficiently highto protect an energy storage device from becoming damaged due toexcessive draining of charge therefrom.

Further advantageously the prescribed SoC threshold may be set to avalue of around 10%, from around 10% to around 20%, from around 20% toaround 30%, from around 30% to around 40% and from around 40% to around50% of an upper limit to the amount of charge that an energy storagedevice is capable of storing.

The controller may be arranged wherein the prescribed SoC threshold isset to a value sufficient to allow the vehicle to travel the prescribeddistance by means of torque provided by the electric machine with theengine off provided a value of driver demanded torque does not exceed aprescribed threshold value.

It is to be understood that in the case of a parallel hybrid electricvehicle an engine start may be required even when the battery SoC ishigh when the electric machine alone is unable to provide the amount oftorque demanded by the driver (driver demanded torque). Thus it maybecome necessary to start the engine in order to meet the driver'storque demand even though the vehicle has sufficient charge to continueoperating in EV mode.

In the case of a series-type HEV, it may also be required to start theengine if an amount of current available from the battery to power theelectric machine is insufficient to meet the value of driver demandedtorque even though the battery SoC may be above the prescribed thresholdfor operation with the engine off under lower (or normal) driverdemanded torque conditions.

Advantageously the controller may be further arranged upon start-up tocontrol the vehicle to start an engine if an ambient temperature isbelow a prescribed temperature threshold.

This feature has the advantage that the controller is able to anticipatethat a driver of the vehicle may require the vehicle cabin heatingsystem to be activated. Thus the controller may control the vehicle tostart the engine in order to provide adequate cabin heating to establisha comfortable environment for the driver.

Further advantageously, the controller may be arranged upon start-up tocontrol a vehicle to start an engine if an ambient temperature is abovea prescribed temperature threshold.

This feature has the advantage that the controller is able to anticipatethat a driver of the vehicle may require a vehicle cabinair-conditioning system to be activated. Thus the vehicle may start theengine in order to provide adequate power to cool the cabin andestablish a comfortable environment for the driver.

According to a further aspect of the invention there is provided ahybrid electric vehicle comprising an engine, an electric machine and acontroller as set out in any of the preceding paragraphs.

The vehicle may be provided with a plurality of electric machines. Oneor more of the electric machines may be operable as a motor. In additionor instead one or more of the electric machines may be operable as agenerator.

In addition or instead one or more of the electric machines may beoperable as a motor/generator.

According to a still further aspect of the invention there is provided amethod of controlling a hybrid electric vehicle having an engine and anelectric machine, the method comprising: upon start-up, controlling theelectric machine to provide torque to drive a vehicle with an engine offif a state of charge (SoC) of an energy storage device is above anEV-start SoC threshold and to start an engine if a SoC of an energystorage device is below the EV-start SoC threshold, wherein the EV-startSoC threshold is determined to be one selected from amongst a valuesufficient to allow a vehicle to travel a prescribed distance before aSoC falls below a SoC minimum level at which an engine is started and avalue sufficient to allow a vehicle to operate for a prescribed timeperiod before a SoC falls below the SoC minimum level.

According to yet another aspect of the invention for which protection issought there is provided a controller for a hybrid electric vehicle(HEV) having an engine and an electric machine, the controller beingconfigured to control the vehicle to operate in a parallel mode in whichthe engine and electric machine provide torque to propel the vehicle andan electric vehicle (EV) mode in which the electric machine aloneprovides torque to propel the vehicle, the controller being arrangedupon start-up to control the vehicle to operate in EV mode if thebattery state of charge (SoC) is above a prescribed SoC threshold and tostart the engine if the battery SoC is below the prescribed SoCthreshold.

According to a further aspect of the invention there is provided ahybrid electric vehicle comprising an engine, an electric machine and acontroller according to the preceding paragraph.

According to another aspect of the invention for which protection issought, there is provided a controller for a hybrid electric vehicle(HEV) having an engine and an electric machine, the controller beingconfigured upon start-up to control the electric machine to providetorque to drive the vehicle with the engine off if the battery state ofcharge (SoC) is above a prescribed SoC threshold and to start the engineif the battery SoC is below the prescribed SoC threshold.

The HEV may be a series-type HEV in which the engine is not configuredto provide motive torque.

According to a still further aspect of the invention for whichprotection is sought, there is provided a method of controlling a hybridelectric vehicle having an engine and an electric machine, the methodcomprising: upon start-up, controlling the electric machine to providetorque to drive the vehicle with the engine off if the battery state ofcharge (SoC) is above a prescribed battery SoC threshold and startingthe engine if the battery SoC is below the prescribed battery SoCthreshold.

According to another aspect of the invention for which protection issought there is provided a method of controlling a hybrid electricvehicle having an engine and an electric machine, the method comprising:upon start-up, controlling the vehicle to operate in EV mode if thebattery state of charge (SoC) is above a prescribed SoC threshold andstarting the engine if the battery SoC is below the prescribed SoCthreshold.

In one embodiment of the invention there is provided a controller for ahybrid electric vehicle (HEV) having an engine and an electric machine,the controller being configured upon start-up to control an electricmachine to provide torque to drive a vehicle with an engine off if astate of charge (SoC) of an energy storage device is above an EV-startSoC threshold and to start an engine if a SoC of an energy storagedevice is below the EV-start SoC threshold, wherein the EV-start SoCthreshold is determined responsive to one or more vehicle operatingparameters.

The one or more vehicle operating parameters may be one or more stateparameters indicative of a current state of a vehicle. One of theparameters may correspond for example to an amount of current beingdrained by an energy storage device of a vehicle at a present time.Alternatively or in addition one parameter may correspond to an amountof current that it is determined that a vehicle is likely to draw duringat least an initial portion of a journey the vehicle is to make. Thisdetermination may be made with or without data in respect of a route avehicle is to take or of a destination of a vehicle.

Within the scope of this application it is envisaged that the variousaspects, embodiments, examples, features and alternatives set out in thepreceding paragraphs, in the claims and/or in the following descriptionand drawings may be taken independently or in any combination thereof.Features described in connection with one embodiment are applicable toall embodiments, except where there is incompatibility of features.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying figures in which:

FIG. 1 is a schematic illustration of a hybrid electric vehicleaccording to an embodiment of the invention; and

FIG. 2 shows a relationship between parameters SoC(min) and SoC(offset).

DETAILED DESCRIPTION

A hybrid electric vehicle (HEV) 100 according to an embodiment of thepresent invention is shown schematically in FIG. 1. The HEV 100 has aninternal combustion engine 121 releasably coupled to a crankshaftintegrated motor/generator (CIMG) 123 by means of a clutch 122. The CIMG123 is in turn coupled to an automatic transmission 124. Thetransmission 124 is arranged to drive a pair of front wheels 111, 112 ofthe vehicle by means of a corresponding pair of front drive shafts 118.The vehicle 100 also has an auxiliary driveline 130 arranged to drive apair of rear wheels 114, 115 by means of an auxiliary driveshaft 132 (orpropshaft 132) and a rear differential 135.

A battery 150 is provided that may be electrically coupled to the CIMG123 in order to allow the CIMG 123 to generate torque when the CIMG 123is operated as a motor. Alternatively the battery 150 may beelectrically coupled to the CIMG 123 to receive charge from the CIMG 123when the CIMG 123 is operated as a generator in order to recharge thebattery 150.

The vehicle 100 is configured to operate in a parallel mode or anelectric vehicle (EV) mode.

In the parallel mode of operation the clutch 122 is closed and theengine 121 is arranged to provide torque to the transmission 124 throughthe CIMG 123. In this mode the CIMG 123 may be operated either as amotor or as a generator.

In the EV mode of operation the clutch 122 is opened and the engine 121is turned off. Again, the CIMG 123 is then operated either as apropulsion motor or as a generator. The CIMG 123 may be operated as amotor in order to drive the vehicle or as a generator in order to effectregenerative braking of the vehicle 100.

The vehicle 100 has a controller 140 arranged to control the vehicle 100to transition between the parallel and EV modes depending on a varietyof parameters associated with the vehicle 100 and driver actions thatwill not be discussed herein.

It is to be understood that when an operator of the vehicle 100initially starts the vehicle 100 the vehicle 100 is controlled totransition from a ‘not ready’ state in which the vehicle 100 is notready to move to a ‘ready’ state in which the vehicle 100 is ready move.By ready to move is meant that if a driving mode of the transmission 124is selected and an accelerator pedal 161 of the vehicle 100 is depressedthe vehicle 100 may be driven.

In the embodiment of FIG. 1 the vehicle 100 is operable to transitionfrom the ‘not ready’ state to the ‘ready’ state by pressing a ‘START’button. It is to be understood that in a non-hybrid vehicle pressing aSTART button (or turning a key to a START position) typically results instarting of the engine.

However in a HEV the engine 121 may not start immediately upon selectingSTART. For example in the case of a mild hybrid vehicle (being a‘stop-start’ vehicle not having an EV mode) pressing the START buttonmay not result in an engine start until the accelerator pedal 161 isdepressed.

In a ‘full’ hybrid electric vehicle 100 according to the presentinvention (a ‘full’ HEV being a HEV having an EV mode in which theengine may be switched off) the controller 140 is arranged to determinewhether to start the vehicle in EV mode or parallel mode upon the STARTbutton being pressed. In some embodiments the controller 140 determineswhether to start in EV mode or parallel mode when a driving mode of thetransmission 124 is selected. In some embodiments the controller 140determines whether to start in EV mode or parallel mode when theaccelerator pedal 161 is depressed.

The controller 140 is arranged to determine the required mode responsiveto the state of charge (SoC) of the battery 150, an ambient temperatureof the environment in which the vehicle is operating and a determinationwhether the vehicle 100 is likely to be able to travel a prescribeddistance in EV mode before requiring a transition to a parallel mode. Insome alternative embodiments the controller 140 is arranged to determinethe required mode responsive to a determination whether the vehicle 100is likely to be able to operate for a prescribed time period in EV modebefore requiring a transition to a parallel mode. By the term ‘operate’is included standing of the vehicle or travel of the vehicle.

In the embodiment of FIG. 1 the controller 140 is arranged to determinehow far the vehicle may travel in EV mode given the current SoC of thebattery 150 and a likely charge drain on the battery 150 whilst driving.If the distance exceeds a prescribed distance and the ambienttemperature is within a prescribed range (being above a lower thresholdvalue and below an upper threshold value) the controller 140 is arrangedto start the vehicle in EV mode. Thus, the engine 121 is not startedwhen the START button is pressed or the accelerator pedal 161 isdepressed.

It is to be understood that the upper and lower temperature thresholdvalues may be selected to bound a temperature range over which it isanticipated that an operator of the vehicle will not require activationof an engine-driven heating or air-conditioning system of the vehicle100. In some embodiments the vehicle 100 is arranged to allow theoperator to set values of the upper and lower temperature thresholdsaccording to his or her preference.

In determining the distance the vehicle 100 is likely to be able totravel, the vehicle is configured to check several vehicle stateparameters indicative of a likely charge drain on the battery 150 whilstthe vehicle 100 is operating. The state parameters include anoperational state of one or more vehicle accessories such as aninfotainment system, an operational state of one or more electricalheating elements such as a front or rear windscreen demist heatingelement, an ambient light level indicating a likelihood that one or morevehicle running lights may require to be activated, an operational stateof one or more vehicle running lights (for example whether runninglights are activated or not, and optionally a setting of the lights suchas side lights only, side lights and headlights, or sidelights and fullbeam headlights), a vehicle loading and a determination whether avehicle is towing a load. Data in respect of vehicle loading may beobtained by one of a variety of different means, for example byreference to a pitch angle of a vehicle, a vehicle suspension system orone or more other parameters or systems. Data in respect of whether avehicle is towing a load may be obtained by reference to whether anelectrical lighting board of a towed object such as a trailer has beenconnected to the vehicle 100 or one or more towing sensors.

In some embodiments the controller 140 is configured to determine avalue of a state of charge offset parameter SoC(offset) being aparameter indicative of an amount of charge the vehicle 100 is likely todraw from the battery 150 in order to travel the prescribed distance.This parameter is indicated in FIG. 2 which illustrates a relationshipbetween a minimum allowable SoC of the battery, SoC(min), the value ofSoC(offset) and a present value of SoC, SoC(present). If SoC(present) isgreater than or equal to a sum of SoC(min) and SoC(offset), the vehicle100 is configured to commence driving in EV mode. If SoC(present) isless than a sum of SoC(min) and SoC(offset) the vehicle 100 isconfigured to start the engine 121 and to operate in a parallel HEV modeor other mode in which the engine 121 is employed to drive the vehicle100.

Embodiments of the invention have the advantage that a more consistentstart-up ‘feel’ may be provided to an operator of the vehicle 100. Thisis because the vehicle 100 is arranged to launch (or ‘initialise’) in EVmode rather than parallel mode whenever conditions permit.

In some embodiments of the invention the vehicle 100 is arranged tomaintain at substantially all times a battery SoC sufficient for EVoperation when required, and sufficient to meet the requirementsdiscussed above for start-up in EV mode.

Thus in some embodiments the vehicle 100 will typically be ‘shut down’(entering the ‘not ready’ mode) at the end of a journey with a batterySoC that is sufficient to allow starting in EV mode and travel of theprescribed distance (or to operate for the prescribed time period)discussed above.

However other arrangements are also useful.

In some embodiments, the prescribed distance may itself be responsive toone or more parameters. In some embodiments the prescribed distance maybe responsive to a geographical location of the vehicle 100. Thus if thevehicle 100 is located in an urban area the prescribed distance maydiffer from that in the case that the vehicle 100 is located away froman urban area. In some embodiments the prescribed distance may be lowerin an urban area. In some embodiments the prescribed distance may begreater in an urban area.

In some embodiments of the invention allowing recharging of the battery150 from an external power source, it is to be understood that it may benecessary to recharge the battery 150 from the external power source inorder to permit start-up in EV mode.

It is to be understood that other arrangements are also useful.

A specific embodiment of the invention has been described as implementedin the parallel-type hybrid electric vehicle of FIG. 1. It is to beunderstood that embodiments of the invention are equally applicable tohybrid vehicles operating in series mode in which the engine 121 doesnot provide driving torque to a driveline or wheels of the vehicle butrather drives a generator to charge the battery to allow a propulsionmotor to drive the vehicle. In some HEVs the generator may be operableto provide electrical power directly to a propulsion motor providingdriving torque to one or more wheels.

Throughout the description and claims of this specification, the words“comprise” and “contain” and variations of the words, for example“comprising” and “comprises”, means “including but not limited to”, andis not intended to (and does not) exclude other moieties, additives,components, integers or steps.

Throughout the description and claims of this specification, thesingular encompasses the plural unless the context otherwise requires.In particular, where the indefinite article is used, the specificationis to be understood as contemplating plurality as well as singularity,unless the context requires otherwise.

Features, integers, characteristics, compounds, chemical moieties orgroups described in conjunction with a particular aspect, embodiment orexample of the invention are to be understood to be applicable to anyother aspect, embodiment or example described herein unless incompatibletherewith.

1-16. (canceled)
 17. A controller for a hybrid electric vehicle (HEV)having an engine and an electric machine and an energy storage device,the controller being configured to start the engine if a state of charge(SOC) of the energy storage device is below a SoC minimum level(SoCmin), the controller being further configured, upon initial start-upof the vehicle in which the vehicle is corm-oiled to transition from, anot ready state to a ready state, to determine whether the SoC of theenergy storage device is below an EV-start SoC threshold (SoCEV-start)instead of SoCmin; if the SoC of the energy storage device is below anEV-start SoC threshold the vehicle is controlled to start the engine,and if the SoC of the energy storage device is not below an EV-start SoCthreshold the vehicle is controlled to start in EV mode, wherein SoCEV-start is greater than SoCmin and. comprises at least one of: a valuesufficient to allow the vehicle to travel a prescribed distance beforethe SoC falls below SoCmin; and a value sufficient to allow the vehicleto operate for a prescribed time period before the SoC falls belowSoCmin.
 18. A controller as claimed in claim 17 wherein a prescribed SoCthreshold is set to a value sufficiently high to protect the energystorage device from becoming damaged due to excessive draining of chargefrom the energy storage device.
 19. A controller as claimed in claim 18wherein the prescribed SoC threshold is set to a value of around 10%,from around 10% to around 20%, from around 20% to around 30%, fromaround 30% to around 40% and from around 40% to around 50% of an upperlimit to an amount of charge the energy storage device, is capable ofstoring.
 20. A controller as claimed in claim 17 wherein a prescribedSoC threshold is set to a value sufficient to allow the vehicle totravel the prescribed distance provided a value of driver demandedtorque does not exceed a prescribed threshold value.
 21. A controller asclaimed in claim 17 further arranged upon start-up to control thevehicle to start the engine if an ambient temperature is below aprescribed temperature threshold.
 22. A controller as claimed in claim17 further, arranged upon start-up to control the vehicle to start theengine if an ambient temperature is above a prescribed temperaturethreshold.
 23. A hybrid electric vehicle comprising an engine, anelectric machine and a controller as claimed in claim
 17. 24. A hybridelectric vehicle as claimed in claim 23 wherein the vehicle is operableby the controller to operate in a parallel, mode in which the engineprovides torque to propel the vehicle and an electric vehicle (EV) modein which the electric machine provides torque to propel the vehicle andthe engine is switched off, the controller being arranged to start theengine if the state of charge (SoC) of the energy storage device isbelow a SoC minimum level (SoCmin), the controller being furtherconfigured, upon initial startup of the vehicle in which the vehicle iscontrolled to transition from a not ready state to a ready state, todetermine whether the SoC of the energy storage device is below anEV-start SoC threshold (SoCEV-start) instead of SoCmin; if the SoC ofthe energy storage device is below an EV-start SoC threshold the vehicleis controlled to start the engine, and if the SoC of the energy storagedevice is not below an EV-start SoC threshold the vehicle is controlledto, start in EV mode, wherein SoC EV-start is greater than SoCmin andcomprises at least one of: a value sufficient to allow the vehicle totravel the prescribed distance before the SoC fails below SoCmin; and avalue sufficient to allow the vehicle to operate for a prescribed timeperiod before the SoC falls below SoCmin.
 25. A vehicle as claimed inclaim 24 wherein in the parallel, mode the electric machine is operableas an electric motor to provide torque to drive the vehicle.
 26. Avehicle as claimed in claim 24 wherein in the parallel mode the electricmachine is operable as an electric generator to charge the energystorage device.
 27. A vehicle as claimed in claim 23 wherein the vehicleis a series-type hybrid electric vehicle.
 28. A method of controlling ahybrid electric vehicle having an engine and an electric machine, themethod comprising: upon initial start-up of the vehicle in which thevehicle is controlled to transition from a not ready state to a readystate, determining whether a SoC of an energy storage device of thevehicle is below an EV-start SoC threshold (SoCEV-start) instead of aSoC minimum level (SoCmin); if the SoC of the energy storage device isbelow the EV-start SoC threshold, controlling the vehicle to start theengine, and if the SoC of the energy storage device is not below theEV-start SoC threshold, controlling the vehicle to start in EN mode,wherein SoC EV-start is greater than SoCmin and comprises at least oneof: a value sufficient to allow the vehicle to travel a prescribeddistance before the SoC falls below SoCmin; and a value sufficient toallow the vehicle to operate for a prescribed time period before the SoCfalls below SoCmin.
 29. A method as claimed in claim 28 furthercomprising, when in a parallel mode, controlling the electric machine asan electric motor to provide torque to drive the vehicle.
 30. A methodas claimed in claim 28 further comprising, when in a parallel mode,controlling the electric machine as an electrical generator to chargethe energy storage device.
 31. A method as claimed in claim 28 furthercomprising controlling the vehicle to assume a parallel mode if a valueof driver demanded torque exceeds a prescribed threshold value.
 32. Amethod as claimed in claim 28 comprising controlling the vehicle tostart the engine if an ambient temperature is below a prescribedtemperature threshold.
 33. A method as claimed in claim 28 furthercomprising controlling the vehicle to start the engine if an ambienttemperature is above a prescribed temperature. threshold.